In cellular communications, the term handover or handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. A basic form of handover is when a phone call in progress is redirected from its current ceil (called a source) to a new ceil (called a target). Such a handover, in which the source and the target are different cells is called an inter-cell or cell-to-cell handover. The purpose of cell-to-cell handover is to maintain the call as the user is moving out of the area covered by the source cell and entering the coverage area of the target cell.
Cellular network operational standards specific cell handover protocols. For example, the Long-term Evolution (LTE) specifies certain handover protocols. In a standard LTE handover procedure, after a user equipment (UE) reports a target cell (or node) downlink (DL) signal quality to Us source node, the source node makes a handover decision. If a handover is to occur, the source node acquires all the relevant information from the target node and delivers it to the UE. This information helps a UE to initiate a Random Access (RACH) process to acquire synchronization with the target node. Subsequently, the target node sends ill allocation and UL timing adjustment to the UE. After receiving the above information, the UE responds with a RRC (Radio Resource Control) Connection Reconfiguration Complete message and the handover is completed.
Although the standard handover procedure described above is adequate in most scenarios, handover failures (HOF) and radio-link failures (RLF) do occur during handover regions, especially when nodes are newly deployed. In some cases the HOF/RLF can be avoided if the handover delay is minimized. For example, this is the case with mobility in a hetrogeneous network in scenarios where a UE approaches a small cell (e.g., picocell or femtocell) at high or at least medium speed. In these circumstances, the UE may experience HOF due to handover delay. Although certain mobility parameters (e.g., Time-to-Trigger) may foe adjusted to compensate for the speed of the UE, some mobility parameters are not cell specific and cannot be adjusted fast enough to accommodate certain operational needs. To resolve these issues and possibly others, a more compact handover procedure is needed with reduced handover delay.